1. Field of the Invention
The present invention relates to a so-called metallic thin film type magnetic recording medium comprising metallic magnetic thin film formed on a non-magnetic support base and a method of manufacturing it. More particularly, the present invention relates to a metallic thin film type magnetic recording medium available for a durable video tape with a longer recording time or a tape for high density magnetic recording, which may be used as a large capacity tape streamer. The present invention also relates to a method of manufacturing such a magnetic recording medium.
2. Description of the Related Art
Conventionally, such a so-called coating type magnetic recording medium is widely known as a magnetic recording tape for recording audio signal and video signal. The coating type magnetic recording medium is manufactured by coating and drying of magnetic paint on non-magnetic supporting base. The magnetic paint may be produced by way of dispersing magnetic powder such as magnetic oxide powder or magnetic alloy powder in a variety of bonding agents such as copolymer of vinyl chloride and vinyl acetate, polyester resin, polyurethane resin, or urethane resin, for example.
Along with growing demand for higher density recording in recent years, so-called magnetic thin film type magnetic recording medium has been introduced. This type of magnetic recording medium comprises a magnetic layer directly formed on a non-magnetic supporting base or via an extremely thin adhesive layer. The magnetic layer is formed using metallic magnetic material such as Co—Ni alloy, Co—Cr alloy, or Co—O by applying a variety of vacuum thin film forming techniques such as a plating method, vacuum vapor deposition method, sputtering method, or ion plating method, or the like.
Because of distinguished coercivity, square ratio, and capability to form a magnetic layer into an extremely thin layer, the above metallic thin film type magnetic recording medium has a superior electromagnetic conversion characteristics in short wave band and can significantly minimizes demagnetization in the recording and loss of thickness during a replay operation as well. Further, unlike the coating type magnetic recording medium, inasmuch as no binder comprising non-magnetic material is present in the magnetic layer, it is possible to enhance density of ferromagnetic metallic particles to be filled therein, thus providing various advantages.
Further, in order to improve electromagnetic conversion characteristics and generate larger output, such a method of obliquely forming the magnetic layer via so-called diagonal vapor deposition method has already been in practical use.
Typically, in the above cited metallic thin film type magnetic recording medium, in order to improve durability and running characteristics, either a protection layer is formed on the magnetic layer or a back layer is formed on the opposite side surface from the surface on which the magnetic layer is formed.
Further, in the metallic thin film type magnetic recording medium, in order to minimize spacing loss in correspondence with the higher density recording, the surface of this magnetic recording medium has been smoothened furthermore.
Nevertheless, after further smoothening the surface of the magnetic layer, an area in contact with a magnetic head expands. The expansion causes friction force to be intensified and shearing force in the magnetic layer to be risen. In order to protect the magnetic layer from such severe frictional sliding condition, a protection film may also be formed over the magnetic layer.
It is known that the protection film can be formed with a carbon film, a quartz (SiO2) film, or a zirconia (ZrO2) film, for example. These films have already been applied to a hard disc. In recent years, among a variety of carbon films, a rigid carbon film (so-called a diamond-like carbon film) formed with diamond structure has widely been utilized as a distinguished protection film. The protection film comprising rigid carbon is formed by applying such a typical sputtering method or a plasma CVD (chemical vapor deposition) method.
In the case of applying the sputtering method, initially, availing of electric field or magnetic field, sputtering gas comprising argon gas is ionized and turned into plasma, and then the plasma is accelerated to hit against a target surface. Target atoms are sputtered out from the target surface to which the plasma particles are collided. The sputtered atoms are deposited on an object to be processed to form a sputter film. However, in the case of forming the rigid carbon film via the sputtering process, it is found that a rate of forming the carbon film is generally slow. Accordingly, this sputtering process is disadvantageous in view of industrial productivity.
On the other hand, in the case of applying the above plasma CVD method, initially, raw material gas for forming the film is subject to chemical reaction thereby generating decomposition or synthesis of the raw material gas by effect of energy of plasma generated by the electric field. Resultant material generated from the chemical reaction is then deposited on the processed object to form the CVD film. The plasma CVD method forms the CVD film much faster than the above sputtering method, and thus, the plasma CVD method is quite promising as an effective means for forming the rigid carbon protection film.
Referring to a plasma CVD processing apparatus shown in FIG. 2, a method of forming a rigid carbon protection film using the plasma CVD method is described below.
The plasma CVD processing apparatus shown in FIG. 2 comprises a vacuum chamber 11 including a cylindrical rotating support body 12 with ground potential, a reaction tube 13 disposed by way of facing the rotating support body 12, and a discharge electrode 14 fitted inside of the reaction tube 13. An end 15 of the reaction tube 13 penetrates the bottom portion of the vacuum chamber 11 in order to introduce reaction gas for forming a rigid carbon film including vaporized aliphatic hydrocarbons such as ethylene or such gas comprising aromatic hydrocarbon vaporized from liquid material such as toluene for example inside of the reaction tube 13 via an end 15 of the reaction tube 13. It is preferred to utilize the discharge electrode 14 that is capable of easily letting gas components through it and evenly generating electric field. Accordingly, it is preferred to configure the discharge electrode 14 with a mesh form for accommodating structural flexibility. Although copper is cited as a typical material, any metal having a reasonably large electric conductivity may also be utilized, which includes stainless steel, brass, and gold, for example.
In the plasma CVD processing apparatus shown in FIG. 2, an object 16 to be processed and formed with a CVD film is guided between a supply roll 17 and a take-up roll 18 by a pair of guide rolls 19 in order that it can continuously run itself along the surface of the above rotating support body 12. When the object 16, which continuously running through the rolls, arrives at a location of the rotating support body 12 corresponding to a position opposite from the discharge electrode 14, plasma is generated between metallic magnetic layer of the object 16 and the discharge electrode 14 to cause reaction such as decomposition or synthesis of raw material gas to be generated. As a result, decomposed or synthesized material generated via the above reaction is deposited in succession to cause the CVD film consisting of such a rigid carbon protection film. Simultaneously, current is grounded via the object 16 and the rotating support body 12 having the ground potential.
As described above, by operating the above plasma CVD processing apparatus shown in FIG. 2, the raw material gas is decomposed by effect of plasma discharge between metallic component present in the object 16 and the discharge electrode 14, whereby forming a rigid carbon protection film.